1. Field of the invention
The resent invention relates to an air-fuel ratio control system for an internal combustion engine which causes burning a lean fuel mixture under specific engine operating conditions.
2. Description of Related Art
In order for internal combustion engines to yield improved fuel economy or fuel efficiency, it has been proved effective to produce a stratified fuel mixture in an combustion chamber and/or to accelerate atomization and evaporation of fuel by means of adjusting a timing of fuel injection so as to achieve, on one hand, improved combustibility of a fuel mixture and, on the other hand, combustion of a fuel mixture leaner than a "stoichiometric" air-fuel ratio, which is an engineering term for an ideally combustible air-fuel ratio, in a specific range of engine operating conditions. Further, in recent years, there have been proposed various closed loop or feedback air-fuel ratio control systems, for determining the oxygen content of exhaust and constantly monitoring the exhaust to verify the accuracy of a fuel mixture setting based on a deviation from a target air-fuel ratio according to a specific engine operating condition, which prohibit burning a lean fuel mixture and burns a fuel mixture at a stoichiometric air-fuel ratio when an air-fuel ratio sensor, such as an oxygen (O.sub.2) sensor for detecting the oxygen content of exhaust. Such a feedback air-fuel ratio control system prevents aggravation of engine performance and deterioration in emission control.
In lean burn engines of this kind, if the lean burning lasts in spite of occurrences of troubles of a means for producing stratified fuel mixture in an combustion chamber and/or a means for adjusting a timing of fuel injection, the engine is continuously operated with a fuel mixture burned at lean air-fuel mixtures without increasing combustibility, which is always undesirable and leads to accidentally burning. For instance, in the case where a sensor is used to specify cylinders so as to adjust timing of fuel injection to the cylinders separately from one another so as to improve combustibility, malfunctions of the sensor disables the control of fuel injection at appropriate timing separately to the respective cylinders. If burning lasts at lean air-fuel ratios under such circumstances, the engine causes burning accidentally and is disabled to operate appropriately.